Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 474

A large number of weeds have evolved races in which the size and shape of
the propagules cause them to be threshed out with the crop seed, particularly
with traditional seed-cleaning techniques (Barrett, 1983). Many seed crops
have such mimics. The problem of seed mimics has been greatly reduced in
the developed countries by modern seed-cleaning procedures and seed certifi-
cation programs.Thus, for example, the once common weed of winter wheat,
Agrostemma githago , is now rarely seen on industrialized wheat farms. In such
cases, technology advanced faster than the adaptive response of the weeds.
Presumably, if improvements in seed cleaning had taken place more slowly,
the size and shape of the weed seeds would have become even more closely
matched to those of their model crops, and seed-cleaning techniques that are
presently effective would be insufficient. Although greatly reduced, the
problem of seed mimic weeds has not been eliminated. Thus, for example,
mimetic Vicia sativa is still found in lentil fields in the USA (Erskine, Smartt &
Muehlbauer, 1994), and Cardiospermum halicacabum has become a problem for
soybean seed producers in parts of the southern USA (Johnston, Murray &
Williams, 1979; Bridges, 1992).
Variation among weed populations of a species implies that best manage-
ment practices will sometimes vary between locations. For example, Cavers
(1985) summarized data on seven biotypes of Panicum miliacium found in
Canada. 'Black' was extremely distasteful to birds, whereas most other bio-
types were attractive to birds and could be expected to suffer high seed preda-
tion if left on the soil surface through the winter. On the other hand, 'black'
was sensitive to some herbicides to which other biotypes were tolerant. Also,
collection of seed during crop harvest could in principle control the several
non-shattering biotypes, whereas this option is not available for 'black'which
shatters easily. As weed management shifts from reliance on herbicides to
tactics that target specific ecological characteristics of weeds, variation in
those characteristics will need to be taken into account.
Managing the adaptation of weed populations
Will weeds adapt to ecological management tactics?
The studies discussed above reveal several points about the differenti-
ation of weed populations. First, substantial spatial variation in ecologically
significant characteristics occurs in many weed species.This variation involves
dormancy, phenology, stature, growth rate, and other characters that affect
the efficacy of various ecological weed management tactics. Second, since
much of the variation in such traits is heritable (Naylor & Jana, 1976; Jordan,